The present invention relates in general to detecting misfires occurring during normal in-use vehicle operation of internal combustion engines and more specifically to identifying the occurrence of individual misfires with reduced false alarms or other errors.
Catalytic converters are used in automobiles to reduce the amount of pollutants in the engine exhaust. When a cylinder misfires so that no combustion or incomplete combustion occurs, uncombusted fuel is introduced into the exhaust which then burns in the hot catalytic converter. The heat from fuel burning in the catalytic converter destroys the catalyst. Thus, it becomes desirable to detect and count engine misfires and signal the operator of the vehicle upon occurrence of excessive misfires so that steps may be taken to protect the catalytic converter.
One of the best techniques for detecting misfires has been to monitor engine crankshaft acceleration, as described in commonly owned U.S. Pat. Nos. 5,044,194; 5,044,195; 5,056,360; 5,095,742; 5,109,695; and 5,117,681, which are hereby incorporated by reference. In these systems, the time to rotate through an angular interval associated with each engine cylinder is measured. Acceleration is calculated based on the time intervals and the acceleration values are processed to determine a power loss value associated with each cylinder. Power loss values are compared with a threshold to detect misfires.
Parameters for successful operation of these misfire detectors are quite demanding. For example, it is desirable to detect a misfire rate of about one or two percent since this rate can adversely affect emission levels. In addition, the identity of the misfiring cylinder associated with each individual misfire must be determined and stored to facilitate later servicing of an engine to correct the condition leading to the misfires. Typically, such diagnostic strategies must have very low false alarm rates, especially when detectors are deployed in large numbers of vehicles.
Misfire detection becomes more difficult as the number of cylinders in an engine increase above four due to the overlapping of power strokes. The overlap causes the acceleration effects of one cylinder to spill over into the acceleration measurement of adjacent cylinders in the firing order. Thus, a misfire in one cylinder may have a "pulling effect" on the acceleration values of the adjacent normally firing cylinders. This increases the likelihood of falsely declaring the adjacent firings as misfires.